This invention is related to internal seals for air bypass ducts on turbofan aircraft engines.
Modern gas turbine aircraft engines are typically turbofan engines in which a first compressor stage(s) drives some of the intake air into the core of the engine as the remaining intake air bypasses the engine core and rejoins the engine air stream at the exhaust. The engine core generally comprises a high pressure compressor, a combustor and a high pressure turbine. A relatively high percentage of air bypasses the engine core in comxerical engines while in military engines a relatively small percentage of air bypasses the core.
All turbofan engines and particularly augmented turbofan engines require numerous pipes, tubes, shafts and other accessory conduits to pass through the bypass duct in order to connect the core engine with the engine case. There is a need to seal these accessory conduits to prevent air from leaking out of the bypass duct. Leakage of air from the duct causes a decrease in aircraft engine operating efficiency.
Since aircraft engines are exposed to thermal growth, vibration and other sources of relative motion, some kind of movable seal is required between the accessory conduits and the bypass duct. A conventional sliding seal assembly used to prevent air leakage from a bypass duct is shown in FIG. 1. FIG. 1 is labeled "prior art".
FIG. 1 shows a typical connection between a core engine combustion chamber and the outside of the engine case. An accessory 10 is sealed by a conventional seal 12 at the bypass duct wall 14. A sliding seal piece 16 fits tightly around the accessory 10. This seal piece is free to slide within its retaining plate 18 that is bolted to the duct wall 14 by bolts 20. These types of seals have been successfully used for many years.
New types of aircraft engines are being developed that achieve higher bypass air pressure and/or higher operating temperatures. These advanced technology engines require improved sealing means. When the conventional seal is used in an engine having a high bypass pressure, air flows between the duct wall lip 22 and the conduit 10 and presses upward on disk 16. This upward pressure causes the retaining plate 18 to lift or bow slightly away from the duct wall 14. This results in increased bypass air leakage from the engine and in a loss of engine efficiency.
Another type of conventional sealing arrangement utilizes a segmented metallic piston ring which is positioned to slide between two flanges on the accessory conduit. The piston ring seals against a cylindrical bore machined or assembled onto the bypass duct wall. Unfortunately, this type of seal arrangement can only be used for circular hole. It is not always practical to use circular holes in the bypass duct considering the variety of accessory conduits, and as a result piston rings cannot always be used. Piston rings also tend to be bulky, heavy and expensive to manufacture.
Improved seals are particularly necessitated by the development of aircraft engines which are designed to be efficient both at subsonic and supersonic aircraft speeds. These engines are capable of varying engine geometry to change their bypass pressure ratios in order to maximize engine efficiency throughout the flight envelope. This type of engine can develop very high bypass air pressures relative to ambient air pressure. This makes leakage from the engine bypass duct both more likely and more serious. In view of the above a new sealing means is required for limiting air leakage at accessory conduits which penetrate duct walls.
It is an object of the present invention therefore to provide a bypass duct sealing device which provides an improved air seal for accessory conduits and is particularly useful for aircraft engines having high bypass air pressures.